The present invention relates to a tape drive. Such a tape drive may form part of printing apparatus. In particular, such a tape drive may be used in transfer printers, that is, printers which make use of carrier-supported inks.
In transfer printers, a tape which is normally referred to as a printer tape and carries ink on one side is presented within a printer such that a printhead can contact the other side of the tape to cause the ink to be transferred from the tape on to a target substrate of, for example, paper or a flexible film. Such printers are used in many applications. Industrial printing applications include thermal transfer label printers and thermal transfer coders which print directly on to a substrate such as packaging materials manufactured from flexible film or card.
Ink tape is normally delivered to the end user in the form of a roll wound onto a core. The end user pushes the core on to a tape spool, pulls a free end of the roll to release a length of tape, and then engages the end of the tape with a further spool. The spools may be mounted on a cassette, which can be readily mounted on a printing machine. The printing machine includes a transport means for driving the spools, so as to unwind tape from one spool and to take up tape on the other spool. The printing apparatus transports tape between the two spools along a predetermined path past the printhead.
Known printers of the above type rely upon a wide range of different approaches to the problem of how to drive the tape spools. Some rely upon stepper motors operating in a position control mode to pay out or take-up a predetermined quantity of tape. Other known printers rely on DC motors operating in a torque mode to provide tension in the tape and to directly or indirectly drive the spools. Some known arrangements drive only the spool on to which tape is taken up (the take-up spool) and rely upon some form of “slipping clutch” arrangement on the spool from which tape is drawn (the supply spool) to provide a resistive drag force so as to ensure that the tape is maintained in tension during the printing and tape winding processes and to prevent tape overrun when the tape is brought to rest. It will be appreciated that maintaining adequate tension is an essential requirement for the proper functioning of the printer.
Alternative forms of known printer tape drives drive both the take-up spool and the supply spool. A supply spool motor may be arranged to apply a predetermined drag to the tape, by being driven in the reverse direction to the direction of tape transport. In such an arrangement (referred to herein as “pull-drag”), the motor connected to the take-up spool is arranged to apply a greater force to the tape than the motor connected to the supply spool such that the supply spool motor is overpowered and the supply spool thus rotates in the direction of tape transport. The supply spool drag motor keeps the tape tensioned in normal operation.
In a further alternative arrangement a supply spool motor may be driven in the direction of tape transport such that it contributes to driving the tape from the supply spool to the take-up spool. Such an arrangement is referred to herein as “push-pull”. The take-up motor pulls the tape onto the take-up spool as tape is unwound by the supply spool motor such that tape tension is maintained. Such a push-pull arrangement is described in our earlier UK Patent No. GB 2,369,602, which discloses the use of a pair of stepper motors to drive the supply spool and the take-up spool. In GB 2,369,602 a controller is arranged to control the energization of the motors such that the tape may be transported in both directions between spools of tape. The tension in the tape being transported between spools is monitored and the motors are controlled to energise both motors to drive the spools of tape in the direction of tape transport.
As a printer gradually uses a roll of tape, the outer diameter of the supply spool decreases and the outer diameter of the take-up spool increases. In slipping clutch arrangements, which offer an essentially constant resistive torque, the tape tension will vary in proportion to the diameter of the spools. Given that it is desirable to use large supply spools so as to minimise the number of times that a tape roll has to be replenished, this is a serious problem particularly in high-speed machines where rapid tape transport is essential. For tape drives that use both a take-up motor and a supply spool motor, the variation in spool diameters can make it difficult to determine the correct drive signal to be supplied to each motor such that tape tension is maintained, and/or that tape is unwound or rewound at the correct rate.
Given these constraints, known printer designs offer a compromise in performance by way of limiting the rate of acceleration, the rate of deceleration, and the maximum speed capability of the tape transport system. Overall printer performance has, as a result, been compromised in some cases.
Known tape drive systems generally operate in one of two manners, that is either continuous printing or intermittent printing. In both modes of operation, the apparatus performs a regularly repeated series of printing cycles, each cycle including a printing phase during which ink is being transferred to a substrate, and a further non-printing phase during which the apparatus is prepared for the printing phase of the next cycle.
In continuous printing, during the printing phase a stationary printhead is brought into contact with a printer tape the other side of which is in contact with a substrate on to which an image is to be printed. The term “stationary” is used in the context of continuous printing to indicate that although the printhead will be moved into and out of contact with the tape, it will not move relative to the tape path in the direction in which tape is advanced along that path. During printing, both the substrate and tape are transported past the printhead, generally but not necessarily at the same speed.
Generally only relatively small lengths of the substrate which is transported past the printhead are to be printed upon, and therefore to avoid gross wastage of tape it is necessary to reverse the direction of travel of the tape between printing operations. Thus in a typical printing process in which the substrate is travelling at a constant velocity, the printhead is extended into contact with the tape only when the printhead is adjacent to regions of the substrate to be printed. Immediately before extension of the printhead, the tape must be accelerated up to, for example, the speed of travel of the substrate. The tape speed must then be maintained at the constant speed of the substrate during the printing phase and, after the printing phase has been completed, the tape must be decelerated and then driven in the reverse direction so that the used region of the tape is on the upstream side of the printhead.
As the next region of the substrate to be printed approaches, the tape must then be accelerated back up to the normal printing speed and the tape must be positioned so that an unused portion of the tape close to the previously used region of the tape is located between the printhead and the substrate when the printhead is advanced to the printing position. Thus very rapid acceleration and deceleration of the tape in both directions is required, and the tape drive system must be capable of accurately locating the tape so as to avoid a printing operation being conducted when a previously used portion of the tape is interposed between the printhead and the substrate.
In intermittent printing, a substrate is advanced past a printhead in a stepwise manner such that during the printing phase of each cycle the substrate and generally but not necessarily the tape, are stationary. Relative movement between the substrate, tape and printhead are achieved by displacing the printhead relative to the substrate and tape. Between the printing phase of successive cycles, the substrate is advanced so as to present the next region to be printed beneath the printhead, and the tape is advanced so that an unused section of tape is located between the printhead and the substrate. Once again rapid and accurate transport of the tape is necessary to ensure that unused tape is always located between the substrate and printhead at a time that the printhead is advanced to conduct a printing operation.
U.S. Pat. No. 6,082,914 discloses a thermal transfer printer comprising an ink ribbon driven between a supply spool and a take-up spool via a printhead. The printhead transfers ink from the ink ribbon to a media, which is also driven past the printhead. Each spool is driven by a separate DC motor and controlled by a controller which detects the back EMF (BEMF) of the motors and controls drive of the motors.
The spools have inertia which is taken into account when determining the rate at which the motors are driven. This is used to calculate the appropriate motor torque during ribbon acceleration and deceleration to allow constant ribbon tension to be maintained.
The requirements of high speed transfer printers in terms of tape acceleration, deceleration, speed and positional accuracy are such that many known drive mechanisms have difficulty delivering acceptable performance with a high degree of reliability. Similar constraints also apply in applications other than high-speed printers, for instance drives used in labelling machines, which are adapted to apply labels detached from label web. Tape drives in accordance with embodiments of the present invention are suitable for use in labelling machines in which labels are detached from a continuous label web which is transported between a supply spool and a take-up spool.